1. Field of the Invention
A process and apparatus is provided for the reduction of microorganisms in a conductive medium using low voltage pulsed electrical energy.
2. Description of Related Art
Reduction of microorganisms in a medium using electricity has been studied for many decades. Most early efforts focused on the reduction of microorganisms in a medium by passage of a high voltage electric current through the medium to generate heat, thereby killing the microorganisms in the medium by pasteurization. The conductive medium was often a pumpable food or beverage, such as milk or water.
Later efforts focused on the reduction of the microorganisms by so-called "nonthermal" pasteurization methods. These methods involve application of a high voltage electric field to the medium in short pulses. The high voltage electric field generates an applied energy of 150 joules/ml or greater and causes death of the microorganisms by electroporation or lysis of the microbial cell membrane. The shortness of the pulse duration attempted to minimize heating of the medium. However, these methods suffer from numerous disadvantages, especially when applied to pumpable foods and beverages. For example, the high voltage electric field when applied to pumpable foods and beverages can cause structural alterations in the food or beverage, adversely effecting the taste and texture of the food or beverage. In addition, such high applied energies are believed to cause the formation of free radicals in foods and beverages, which compounds are considered to cause or promote cancer. Further, the equipment necessary to generate such high applied energies requires an electrical energy on the order of 100 kV/cm. Furthermore, such methods do not appear to kill all types of microorganisms, such as molds and yeast.
Experiments have been conducted in the prior art using low voltage electric fields. However, these electrical energy applications were considered to be unsatisfactory because they were not deemed to cause irreparable damage to the microorganisms.
As examples of the prior art, reference is made to the following U.S. patents, whose teachings are incorporated by reference: U.S. Pat. No. 4,917,785; U.S. Pat. No. 4,957,606; U.S. Pat. No. 5,026,484; U.S. Pat. No. 5,037,524; U.S. Pat. No. 5,464,513; U.S. Pat. No. 5,514,391; U.S. Pat. No. 5,630,915; U.S. Pat. No. 5,766,447; and the following publications: Bai-Lin Qin et al., "Inactivating Microorganisms Using a Pulsed Electric Field Continuous Treatment System", IEEE Transactions on Industry Applications, Vol. 34, No. 1, January/February 1998; Karl H. Schoenbach et al., "The Effect of Pulsed Electrical Fields On Biological Cells", paper presented at EPRI/Army PEF Workshop II, Chicago, Ill. on Oct. 10-11, 1997; K. H. Schoenbach et al., "Effect of Pulsed Electric Fields on Micro-organisms: Experiments and Applications, paper presented at EPRI/Army PEF Workshop II, Chicago, Ill. on Oct. 10-11, 1997; and Karl H. Schoenbach et al., "The Effect of Pulsed Electric Fields on Biological Cells: Experiments and Applications", IEEE Transactions on Plasma Science, Vol. 25, No. 2, April 1997.